1. Field of the Invention
The present invention relates to a method and related communications device, and more particularly, to a method of handling a point-to-multipoint (p-t-m) Multimedia Broadcast Multicast Service (MBMS) service in a wireless communications system and related communications device that prevent unnecessary system resource waste and power consumption.
2. Description of the Prior Art
The third generation (3G) mobile telecommunications system has adopted a Wideband Code Division Multiple Access (WCDMA) wireless air interface access method for a cellular network. WCDMA provides high frequency spectrum utilization, universal coverage, and high quality, high-speed multimedia data transmission. The WCDMA method also meets all kinds of QoS requirements simultaneously, providing diverse, flexible, two-way transmission services and better communication quality to reduce transmission interruption rates. To enhance multimedia functions in the 3G mobile telecommunications system, a protocol specification developed by the 3rd Generation Partnership Project (3GPP) provides Multimedia Broadcast Multicast Service (MBMS). MBMS is a point-to-multipoint bearer service and is established on an existing network architecture of the Universal Mobile Telecommunications System (UMTS), utilizing Internet Protocol (IP) packets as a medium. Thus, MBMS allows a single source entity to transmit data to multiple user entities simultaneously.
Since the 3GPP has thoroughly defined operations and architecture of MBMS in the related specifications, the operations and architecture of MBMS are described only briefly hereinafter. For the universal mobile telecommunications system (UMTS), the 3G communications system comprises User Equipment (UE), the UMTS Terrestrial Radio Access Network (UTRAN), and the Core Network (CN). Communications protocols utilized include Access Stratum (AS) and Non-Access Stratum (NAS). AS comprises various sub-layers for different functions, including Radio Resource Control (RRC), Radio Link Control (RLC), Media Access Control (MAC), Packet Data Convergence Protocol (PDCP), and Broadcast/Multicast Control (BMC). The sub-layers mentioned, and their operating principles, are well known in the art, and detailed description thereof is omitted. For MBMS, the Core Network further comprises the Broadcast Multicast Service Center (BM-SC) that is not only a newly added network node but also an entry point for receiving external data, e.g. data provided by a content provider. The BM-SC manages provision, delivery, announcement, authorization, and storage of related parameters for MBMS services, and transmits corresponding information to the Gateway GPRS Support Node (GGSN). In the MBMS system architecture, the GGSN plays a role as an entry point for IP multicast traffic, and routes MBMS data to the Serving GPRS Support Node (SGSN) via a proper GPRS Tunnel (GTP). More than one SGSN can exist under the GGSN. The SGSN performs MBMS Bearer Service control functions to accurately transmit data packets to the UTRAN, allowing multiple UEs, which order the same MBMS service in the same UTRAN, to receive the same MBMS data. The SGSN can comprise more than one Radio Network Controller (RNC). The RNCs are responsible for efficiently allocating radio resources for the UEs according to the number of UEs counted by the Node-Bs (NB). The RNC comprises more than one NB that is responsible for transmitting or receiving radio waves to or from the UEs that can be mobile phones or PDAs.
According to the protocol specification developed by the 3GPP, MBMS offers two service modes: Broadcast mode and Multicast mode. When interested in a specific MBMS service, the UE needs to order the BM-SC for the MBMS service by establishing a service agreement including specific service provision phases. These two service modes have different service provision phases. The service provision phase of Broadcast mode includes Service announcement, Session Start, MBMS notification, Data Transfer and Session Stop. The service provision phase of Multicast mode includes Subscription, Service announcement, Joining, Session Start, MBMS notification, Data Transfer, Session Stop and Leaving. For realizing customized services, Joining and Leaving sessions utilized in Multicast mode enable the BM-SC to execute authorization and payment recording for the UEs at the Joining session. The service provision phases mentioned above are well known in the art, and will not be described in detail.
From the standpoint of the RRC, all logical data communication exchange channels, be they for providing data transmission exchange to the UE or for providing RRC layer control signal transmission exchange, are defined in the context of a Radio Bearer (RB). In the UE end, the RB comprises one unidirectional, or a pair of, uplink/downlink logic data transmission exchange channels. In the network end, the RB comprises one unidirectional, or a pair of, uplink/downlink logic data transmission exchange channels.
When providing the MBMS service, the UTRAN may obtain the number of the UEs ordering the MBMS service via a counting procedure, and can adopt either one of the following two transmission modes; point-to-point (p-t-p) transmission and point-to-multipoint (p-t-m) transmission. The UTRAN utilizes the p-t-m transmission to transfer data if selecting the Broadcast mode. On the contrary, if the Multicast mode is selected, the UTRAN utilizes either the p-t-p transmission or the p-t-m transmission to transfer data according to the number of the UEs. In the p-t-p transmission, transmission between the UEs and the UTRAN utilizes a dedicated-channel, and thereby the Node-B establishes one dedicated channel for one UE to transmit data and configuration. Compared with the p-t-p transmission, the p-t-m transmission allows the CN to provide the data and the configuration for multiple UEs in the same cell simultaneously.
In the protocol specification developed by the 3GPP, three logical channels are defined to transmit corresponding MBMS information: MBMS point-to-multipoint Control Channel (MCCH), MBMS point-to-multipoint Traffic Channel (MTCH), and MBMS point-to-multipoint Scheduling Channel (MSCH). MCCH is utilized to transmit a corresponding MBMS configuration, including MBMS Neighboring Cell Information, MBMS Radio Bearer Information, MBMS Service Information, and other information. MTCH is responsible for transferring service data to the UEs. MSCH is used for notifying the UEs with scheduling information of MBMS services. With MSCH, the UEs is allowed to execute reception only when data begins to be transferred, needless to keep on a receiving state during the whole service process. Furthermore, the three logical channels are mapped onto a transport channel, Forward Access Channel (FACH), and the FACH is mapped onto a physical channel, Secondary Common Control Physical Channel (S-CCPCH). In general, before obtaining the MBMS configuration, the UE first has to receive System Information block type 5 or 5bis through S-CCPCH, and detect whether an MCCH configuration carrying the above-mentioned information is included in the System Information block type 5 or 5bis.
According to the subclause 8.7.3 of the 3GPPTS 25.331 V6.8.0 RRC protocol specification, the UTRAN informs the UE with MBMS MODIFIED SERVICE INFORMATION message and other related control information through a Notification Procedure when a change applicable for one or more MBMS services the UE has joined. Additionally, different channels are used to transmit the above-mentioned information to the UE in accordance with a radio resource control(RRC) state of the UE. If the RRC state of the UE is CELL_DCH state, the UTRAN transmits MBMS MODIFIED SERVICE INFORMATION message via Dedicated Control Channel(DCCH). If the UE operates in other RRC states, such as URA_PCH, CELL_PCH, CELL_FACH state or Idle mode, MCCH is used to transmit MBMS MODIFIED SERVICE INFORMATION message. MBMS MODIFIED SERVICE INFORMATION message includes an information element (IE), “MBMS required UE action”, which indicates an action the UE has to execute.
The IE “MBMS required UE action” may carry different context of instructions based on the channels. For example, when MCCH is selected to transmit MBMS MODIFIED SERVICE INFORMATION message, the IE “MBMS required UE action” includes six selections of instructions: “None”, “Acquire counting info”, “Acquire counting info-PTM RBs unmodified”, “Acquire PTM RB info”, “Request PTP RB”, and “Release PTM RB.” “Release PTM RB” indicates that the UE has to release a resource used for receiving an MBMS service as well as one or more radio bearers. Oppositely, when DCCH is selected, the IE “MBMS required UE action” includes three selections of instructions: “None”, “Acquire PTM RB info” and “Request PTP RB”. As can been seen as above, the instruction “Release PTM RB” is not a selection for the IE “MBMS required UE action” in the DCCH case. That is, the UTRAN has no way to require the UE operating in CELL_DCH state to release the corresponding radio bearer(s) for stopping reception of the p-t-m MBMS service. The UE may keep on reception even when the p-t-m MBMS service program finishes at Session Stop. In this situation, the UE wastes power and system resources in reception unable to be stopped.